High-density multi-chip modules containing many integrated circuits (ICs) are typically produced using photolithographic masks to define the circuit. The component ICs on the substrate are interconnected using conductive traces only about 10 .mu.m wide, which are disposed on multiple conductive signal planes. The production of high-density multi-chip modules, especially those with finer traces, is prone to defects arising either during the fabrication of the multi-chip module layers or during the final assembly of the module. Interconnection of all elements comprising the circuit must be correctly defined by the masks and fabricated without defects to achieve proper operation of the device.
Although the prior art does not include a method for making substantial modifications to the circuit design of a multi-chip module following its assembly, various repair schemes have been developed to correct minor defects in exposed conductors detected during the fabrication of a conductor/dielectric signal layer. For instance, a break in a conductive trace in an exposed signal layer can be bridged by the localized deposition of a metal, such as tungsten, using a laser or focused ion beam. Similarly, short circuits can be corrected by blowing off unwanted metal, again using a laser or a focused ion beam. If the defects are too numerous or too large, it may be more practical to scrap the substrate, since repair would be too costly.
When defects arise in the signal layers during the final processing steps or during the assembly stage of the multi-chip module, repair of the module becomes more cost effective. In this case, a method is required that can effect repairs by rerouting the defective conductor sections through alternate (redundant) conductor paths that may lie buried under other signal layers. Circuit rerouting can be achieved by the use of switchable connections, which are formed during the fabrication of the signal layers and remain nonconductive until activated by a laser. A repair method that uses switchable connectors (and switchable disconnectors) is disclosed in co-pending, commonly assigned U.S. patent application Ser. No. 07/321,911, filed Mar. 10, 1989, entitled, INTEGRATED CIRCUIT HAVING REROUTABLE CONDUCTIVE PATHS. The switchable connections are one-time fusible links, which effect a permanent change in the circuit when activated. The switchable connections must be process compatible (i.e., they cannot be accidentally switched by any of the processing steps or procedures during the fabrication of any of the layers) on the substrate and also should be insensitive to rework procedures at a neighboring site. Such switchable connections are not limited to repair of defects, but can also be used to modify the design of a circuit defined on a substrate.
Methods and structures for modifying a conductive path within a substrate have been developed in the prior art. For example, a selectively switchable connection between conductors on a substrate and a method for making such a connection are disclosed in U.S. Pat. No. 4,681,778. This patent teaches that a nonconductive film formed of spaced apart metal columns or islands can be deposited between two conductors on the substrate of a silicon chip. Since the metal columns are not in electrical contact with each other, the film acts like a dielectric. Whenever it is desired to change the film from a nonconducting to a conducting state in order to provide electrical continuity between the two conductors, a laser beam is locally focused on the film, melting the metal columns so that they fuse together, thereby creating a conductive path.
In order to assure that full continuity between the two conductors is achieved, all the metal columns along a continuous path between the conductors must be melted by the laser beam. This may require multiple scans of the film with the laser beam. In addition, if several conductors that include regions of the dielectric film are closely spaced, care must be taken to avoid scanning the laser onto an adjacent region, where it may melt other columns, creating an undesired continuity between other conductors. Furthermore, the integrity of a polymeric dielectric material used as an intermetallic insulator in the device may be locally compromised due to excessive heating during the process of fusing the metal columns.
In consideration of the foregoing problems, it is an object of the present invention to selectively produce electrical continuity between two conductors disposed internally within an insulating coating, on a multi-chip module substrate or in other types of ICs. It is a further object to provide a structure and method for selectively connecting two conductors by localized heating of a very small region adjacent each conductor. Yet a further object is to selectively change a nonconducting region separating two conductors to a conducting state without significant change in the volume at the site. Another object of this invention is to effect this change without compromising the integrity of the insulating overlayer present. Yet another object of this invention is to provide a switchable link that is optically tuned so as to have minimum reflectivity at a specified laser wavelength. These and other objects and advantages of the present invention will be apparent from the attached drawings and the Disclosure of the Preferred Embodiments that follow.